A research project proceeds through four sequential phases P1, P2, P3, and P4 with the following probabilities: P(P1)=0.9, P(P2∣P1)=0.8, P(P3∣P1∩P2)=x, and P(P4∣P1∩P2∩P3)=0.7. If P(P1∩P2∩P3∩P4)=0.2016, find the value of x.
a) 0.32
b) 0.60
c) 0.40
d) 0.28